Tuesday, September 18, 2012

This past weekend (September 15-16, 2012) was the 2nd part of the annual ARRL 10 GHz and up contest and we decided to use one of the highest-available amateur bands - the one known in the FCC rules as "275 GHz and up." Actually, this covers a lot of territory including submillimeter radio frequencies and far infrared wavelengths, but the part that we are more interested in is that for which most of us are equipped to detect directly - light.

We've done this before, managing to have spanned 107 miles (173 km) on several occasions and even 173 miles (278 km) (read about those efforts here - link) so we weren't going to break any of our own DX (distance) records, but it's fun to do this, anyway - and it gave us an excuse (as if we really needed one...) to go out and test some new gear that had not yet been tested over anything but relatively short (20 km or so) paths.

The two locations for the stations were about 96 miles (154 km) apart with Ron and Elaine Jones (K7RJ and N7BDZ) being at the far end at an elevation of about 5600 ft (1700 m) ASL near Park Valley, Utah in the extreme northwestern corner of Utah, a few miles from where the U.S. Transcontinental Railroad was joined for the first time in 1869 and only a few hundred meters away from the historic stagecoach route that paralleled part of that later railroad. Along with friends Gordon (K7HFV) and Gary (AB1IP), I was closer to home at about 9300 feet (2830 m) near a minor protuberance known as "Bountiful Peak" about 10 miles (16 km) north of Salt Lake City. As it turns out, the path is a grazing one and were it not for the slight refraction of the Earth's atmosphere, it may not even quite be "line of sight."

We'd tried this same path during the first weekend of the 10 GHz and up contest but the thick veil of smoke from wild fires elsewhere in the western U.S. prevented a successful contact - although our light beam was occasionally just visible to the binocular-aided eye in Park Valley. This time, however, the air was reasonably clear, only somewhat hazy from the still-burning fires: Since we "almost" made contact a month ago we were confident that this time we would have no problems.

Figure 1:
The high-power red LED shining to the north-northwest.
The lights of Layton, Utah and surrounding communities
may be seen way below, in the background! The dot at the terminus
of the red shaft of light is the light from Ron's end of the path.Click on the image for a larger version.

Soon after we arrived on site Ron shone a 500,000 candlepower halogen spotlight in our direction and immediately we noted a lone, flickering, yellow-red dot in the blackness "above" the last ribbon of visible lights from the populated areas of Layton and Ogden about a mile (1600 meters) in elevation below us. Using this as a visual reference I swung my high-power LED in his direction, using the Rayleigh-scattered shaft of red light as a guide, and immediately Ron reported that it was easily the brightest light visible: Considering that there were only a small handful of lights visible from his dark, rural location, anyway, this wasn't saying much, but if anyone where to have dropped by and looked in that direction they would have seen the bright, red light and asked, unprompted, "What's that?!?"

Using our light as a guide Ron immediately fine-tuned his pointing and soon, a very obvious red light appeared in the darkness. Initially starting out with the lower power 3-watt LED he soon switched to the much higher-powered 20-30-ish watt LED and the red dot in the distance was even more striking than before. The dot at the end of the red shaft of light in the above picture was from Ron's LED.

Soon after we brought our transmitters up to full power we reduced them again to 1/4-1/15th as each other's signals were strong enough that there was noticeable distortion in the received audio - and it also allowed us to run full-duplex (e.g. both sides being able to send and receive simultaneously) without intercepting as much of our own, scattered transmit light and causing acoustic feedback between our speaker and microphone.

This was the first actual "long distance" test of the Phlatlight-based optical transmitter - these using CBT-54 LEDs and permitting a 20dB improvement on the audio received at the far end. This also was the first test of some APD (Avalanche PhotoDiode) based optical receivers that I'd built some time ago (see the link at the bottom of the page) so we set about reduce each other's LED currents to do a sort of "limbo" dance - that's to say we wanted to answer the question "How low can we go?"

It immediately became apparent that even though we could read the Phlatlight modulators' current with a resolution of 0.1 amp, this was still too coarse when we got down to the lowest readable current and were still able to hear each other, so Ron switched to the older 3 watt Luxeon on which the LED current could be measured and adjusted down to the single digits of milliamps. As it turned out, speech was copyable - with some difficulty - down to the 40-50 milliamp range with the old receivers but the APD receivers extended this down to around 20 milliamps - an approximately 6-10dB improvement, a number that agreed reasonably well with what had been calculated using similar measurements done at home on my "Photon Range" using a very dim LED and test receivers.

Practically speaking this meant that at full power with the Phlatlight LEDs we had about 50dB of excess signal at the output of the receivers as compared to the minimum possible signal level using baseband speech and the "naked" ear. Switching to MCW (tone-modulated Morse code) we could extend this by another 6-10dB and the the use of narrowband digital signalling techniques (such as WSPR or QRSS CW - very slow Morse) could have extended this by even another 20 dB or so. The implication of this is that, in theory, we could communicate over that distance with only a milliamp or two of LED current!

Figure 2:
This time, a high-power green LED!Click on the image for a larger version.

Satisfied with our tests I switched to a green CBT-54. Interestingly - but not too surprisingly - Ron reported that subjectively the green LED wasn't really any brighter than the red had been. On previous tests at much shorter distances (a few 10's of miles/km) the green far outshone the red owing to the fact that the human eye is at least 5 times as sensitive to green than the wavelength of red LED that we were using. For these distances the atmospheric attenuation was sapping the vast majority of our light since the shorter (green) wavelengths are attenuated at a far higher rate than the longer ones, a fact that explains red sunsets and that we observed, at the beginning of our testing, that his white, halogen spotlight appeared to us as distinctly yellow/red in color.

The silicon photodetector didn't fare any better since it had far less sensitivity at green than red, the two factors (atmospheric and the Si sensitivity) adding up to between 20 and 30dB in degradation - assuming that the subjective measurement of "equal" brightness between red and green was correct. As it turns out the degradation was probably far greater than that as the APD-based receiver could hardly detect voice at all, but this may have been also due, in part, to the fact that the gain of a standard APD drops off precipitously with shorter wavelengths and that it was likely not focused properly for green light due to chromatic aberration of the Fresnel lens! In retrospect we should have switched to a receiver with a larger, "non-APD" detector - and thus less sensitive to misfocusing due to chromatic aberration.

In addition to using high-power LEDs, we also exchanged 2-way communications using plain, ordinary, cheap low-power red LED laser pointers. The signals were far weaker - mostly owing to the lower optical power of the laser pointer - but each other's lasers were visible to the naked eye over the distance. Because of the combination of the laser's (relatively) coherent light and its small exit aperture (small beam diameter) the scintillation (fading) on the laser-based link was terrible while on the LED-based link it was only just noticeable. Some of the methods and techniques to communicate using laser pointers may be found in the September 5th entry of this blog.

After several hours of standing around in the dark on the mountain, we decided that it was getting early (approaching 2 AM!) and packed things up and made our way down the mountain.

Overall, it was a fun little jaunt giving us a healthy dose of nerdiness... enough to last for a few weeks, anyway!

Afterward:

While we run these tests, we'll often play something from portable audio players so that we have a continuous source of sound. In this case, one of the audio sources that I used was from a Soldersmoke podcast.

A "Cheap" Optical Transceiver lens assembly - If
you really want to improve your receive sensitivity, the best way to do
this is with a large lens. This article describes how one could use
inexpensive foam-core poster board to make an assembly that will focus the distant
light on the detector diode of an optical receiver.

Thursday, September 13, 2012

For a follow-on article in this series, see Part Four (link)for a discussion of how the voting receiver system works.

In parts One and Two the general overview of a "synchronous" (or "simulcasting") and voting repeater system was discussed. In a nutshell:

Both repeaters operate on the same frequency saving spectrum and simplifying the system's use since the user doesn't have to remember which particular frequency of a "normal" linked system covers a certain area best.

The coverage of the two repeaters overlaps to a degree.

Because of precise frequency control, the two transmitters don't really clobber each other in overlap areas, particularly in a moving vehicle.

Because of voting receivers and multiple transmitters, the users can seamlessly move between coverage areas with no intervention on their part.

The total coverage is greater than the sum of the parts owing to the increased likelihood of one or another site hearing the user and/or being heard - particularly if in an area where coverage is spotty to one or both sites.

Originally (back in the late 90's) the idea was to frequency-convert the received signals from the 2-meter frequency to a subcarrier-baseband and send them to the main site where they could be voted upon and then a master modulator would then ship back (via a microwave link) a subcarrier which was then up-converted to the transmitter frequency.

The details were worked out and some of the equipment was actually built and tested - and it worked! However, the magnitude of the task bogged things down and one thing led to another and the project languished - until 2009.

By then I'd already put together 2 voting systems and one multi-transmitter synchronous system (using GPS frequency references) and had other ideas on how to do things a bit more simply which translated to "being more likely to get completed!" The project got underway in earnest in mid-July of 2009 where the plans were re-draw and tasks divided as appropriate.

Instead of building the transmit and receive gear from the ground up it was, instead, decided to modify off-the-shelf GE MastrII radio gear to fit the bill. This equipment is readily available on the surplus market and the individual pieces could be used with little or no modification - which meant that spares of those same pieces (receiver, transmitter, power amplifier, etc.) could be kept on hand as spares! What's more, for the most part these units used common, off-the-shelf parts (resistors, capacitors, transistors) and were thus field-repairable now and for the foreseeable future. Finally, a lot of information is available on these radios on the web so if, in the future some trouble shooting is required, there's plenty of advice to be had online.

What modifications were required to the radios were fairly simple:

Instead of a standard crystal module (called an "ICOM" by GE) a simple, plug-in module (using a "gutted" ICOM) was plugged into the exciter instead. This was connected via coax to an external module that provided the low frequency (at 1/12th of the transmit frequency) at the precise frequency.

Transmit audio was fed into the subaudible tone input port. This was done because it did not have the highpass and lowpass filters that the normal microphone inputs had: We would do the high/low pass filtering externally!

There were some additional modifications done to provide interfacing to the rest of the system - namely an outboard de-emphasis, a low-pass filter and a switchable notch filter (for a "quirk" we later discovered) but these were mounted on the backplane - a more-or-less passive board that would likely never require replacement! Pretty much everything else was "stock" and could be tuned up and adjusted according to the original manuals!

Transmit frequency control:

The most important aspect of a multi-transmitter (simulcasting) repeater system is that the transmitters be where they are supposed to be, frequency-wise! While there are several ways of doing this, we took a somewhat unique approach.
A standard transmit crystal (at 1/12th of the VHF transmit frequency) was ordered and placed into an "EC" type ICOM. This is, in effect, a self-contained oscillator module that has provisions to be frequency-controlled with an external voltage. This module is completely standard and off-the-shelf and it could be plugged into any GE MastrII VHF transmitter and work normally.

In our case, however, this "EC" module is plugged an external module - called a "Disciplined Oscillator" - that takes the crystal frequency (which is 12.2183333 MHz for a 146.620 MHz transmit frequency) and locks it to a reference based on a 10.0 MHz oven-controlled crystal oscillator. This is done by synthesizing an audio frequency, using a PIC microcontroller clocked to the 10 MHz oscillator, that has a resolution of a few parts per billion and with a bit of dividing, mixing and comparison has the result of locking the 12.2183333 MHz oscillator to the 10 MHz reference to within a tiny fraction of a Hz. Essentially, the frequency accuracy is that of the 10 MHz oscillator!

The 10 MHz oscillator is an oven-controlled crystal oscillator (OCXO) pulled from scrapped satellite gear and is well-aged (made in about 1990) and has a stability of about 10E-8 - within 1 Hz or so at the 2 meter transmit frequency. This OCXO also has an external voltage control tuning line that is under control of the PIC microcontroller and with it, the 10 MHz frequency (and thus the transmit frequency) can be tweaked to set each transmitter on the desired frequency - which also means that the frequency difference between the two transmitters may be precisely controlled. In the nearly 3 years since the system was made operational we've observed that the transmitters have stayed within about 1 Hz of their intended frequencies relative to each other over the course of the temperature excursions during the year!

This "Disciplined Oscillator" module also has another function that, since it was computer-based, was easy to implement, and that's as a simple dual cross-band repeater. On Scott's (the remote site) it simply cross-bands the 2 meter receiver to the 70cm link transmitter - taking care of thinks like proper IDing, timeout timers, etc. and it also takes the 70cm link receiver and controls the 2 meter transmitter coming back the other direction: Both operate independently of each other...

Squelch control and voting:

It also does one more thing: COS (Carrier Operated Squelch) signalling. The 146.620 repeater is one of the few repeaters in the area that does not have a subaudible tone requirement, this being because it's an "open" repeater and that extreme care is taken at all receiver sites to keep the receive frequency as clean as possible - a task that is arguably easier since the demise of analog television in the U.S.!

Since the Scott's Hill transmissions are relay to/from the master site via a UHF link there would be an extra squelch tail (the "ker" in "ker-chunk") if the loss of a signal at Scott's were signalled simply by its UHF transmitter being keyed/unkeyed. Instead, the loss of squelch is signalled by the appearance of a strong, 3.2 kHz tone sent over the link which performs two functions:

It signals to a decoder at the master site that the squelch as closed at the other end.

It signals to the voting controller at the master site that the signal being received is "bad" and should NOT be used.

(This 3.2 kHz tone is "notched" out and its brief appearances in the system audio are not heard by the users.)

So, what happens if a user's signal into Scott's is dropping rapidly in and out? As the squelch opens and closes, the tone is turned off and on (tone on = squelch closed/signal dropout.) When the tone is turned on the voter disqualifies this tone, but if that same user is getting into the other receiver (at the master site) then this tone will guarantee that the signal from that receiver will be used, instead.

There is a short "hang time" on the link transmitter which means that when an input signal disappears from the 2 meter receiver at Scott's, the tone will turn on instantly, making the master site ignore the input, and then the UHF link transmitter signal will drop and in this way, the extra squelch tail from the UHF link transmitter dropping is never heard by the user.

As it happens, the signals coming the other way (from the master site to Scott's over the UHF link to be retransmitted on 2 meters) also use this 3.2 kHz tone - this time, to control the Scott's VHF transmitter. In this case, however, the activation of the tone starts the "Unkey" sequence at Scott's allowing time for the disciplined oscillator to put an extra "beep" on the transmitter (so that users know which transmitter they are hearing!) and then unkey the VHF transmitter.

Since the 3.2 kHz tone being sent to Scott's occurs just before the master site's 2 meter transmitter unkeys, it's possible to set the timing so that both sites unkey at precisely the same time: If both site's didn't unkey simultaneously, many users would be annoyed by the presence of an extra squelch tail if they could, in fact, hear the "other" transmitter hanging in there for a short time!

At the master site:

As it turns out, the master site's interfacing a was bit easier... sort of... This repeater's master site is actually split, with the receiver and antenna being several hundred feet away from the transmitter, this being done to put it farther away from the megawatt of RF being emitted from all of the TV and Radio transmitters on the main site! It is connected via transformer-coupled cables (for lightning protection) and has operated with minimum maintenance since the early 1980's.

Since we already had on-hand local COS (squelch) and audio from the receiver, there was no need for the tone signalling schemes of the remote site but, instead, the audio and COS lines could be input to the voter. There was a problem: The "local" receive audio was "too" good!

The way the voter works is that it analyzes mostly the audio above 2.5 kHz and of the receivers being compared, it is the receiver with the MOST audio above 2.5 kHz that is considered as being the one with the worst signal. The reason for this is pretty simple: As an FM signal gets weaker, it gets noisier, so it stands to reason that given two otherwise identical signals, the one that is also noisier will have a total signal level that is higher - particularly at higher audio frequencies.

The problem was that the audio from Scott's had already passed through a radio link which tended to scrape off the audio above about 3.5 kHz or so while the "local" audio, being coupled via wire, had no such low-pass filtering, so we had to add some. What was happening is that the "local" audio - with its additional "highs" (as compared to the audio from Scott's) was being considered to be "bad". By removing those extra high-frequency components and making the two audio signals pretty much equal we were able to make the more-or-less directly comparable.

Next time: A bit more about the voting controller and some of the remote control/monitoring capabilities.

Wednesday, September 5, 2012

Sending voice over light is nothing new. The first wireless voice communications system - using light - was the PhotoPhone, demonstrated in 1878 by Alexander Graham Bell - a full 25 years before Fessenden demonstrated the same feat using radio waves. To be sure, optical communications has certain practical limitations, namely the blinding presence of the sun and the occasional opacity of the atmosphere due to weather, but it's still a fascinating and fun topic of discussion.

I'm one of those people who find wireless communications of any sort to be interesting and I have a particularly keen fascination with optical wireless communications - that is, using "radio waves" that I can see with my own eyes.

For short-range experimentation it's hard to beat a cheap laser pointer - and here is a bit of info on how one might go about this.Modulating the laser pointer:

The laser pointer consists of a laser diode and like any diode, it has a maximum current rating that should be observed with more caution than its voltage. What this means is that you cannot connect a laser diode to any sort of battery and expect it to work properly: Too little voltage and it won't lase while too much voltage, it will never lase again! What is needed is a simple circuit that limits the amount of current fed into the laser diode to a safe level and fortunately, cheap laser pointers always have something that does this.
Increasingly, cheap laser pointers simply rely on a combination of a simple circuit (or even a single resistor!) and the internal resistance of the battery powering it to keep the laser current at a safe level and since a laser pointer already has the necessary parts, why not use them?

In my opinion, one mistake that I often see on web pages that describe the modulation of a laser pointer is to attempt to modulate by varying the voltage/current of operation - typically using a transformer in series with the power source. There are several things wrong with this:

It's not certain how far down in current one can go before the laser drops out of its "laser" mode or how high one can go before it gets "blowed up."

Laser current versus output isn't terribly linear which means that distortion can occur.

With the min/max current uncertainty, one can't fully modulate the laser's output safely which means that the audio on the beam will be somewhat "quiet" - something that reduces the efficacy of the link!

The better way to modulate a laser is to simply turn it on and off using Pulse With Modulation (PWM) and taking advantage of the circuit already present to safely operate the laser from its intended power source - say, a pair of AAA cells (or 3.0 volts.) While more complicated than simply putting the laser's power supply in series with a transformer, it's pretty much bulletproof and can sound pretty darn good!

Figure 1: Laser transmitter/receiver by K7RJ.For a diagram of this unit, see Figure 4 at the bottom of this page.Click on the image for a larger version.

A simple circuit to do this may be found in the diagram in figure 4 at the bottom of the page..

I won't take credit for this circuit which was thrown together by my friend Ron, K7RJ. When built, this circuit was intended to be quick and easy and high performance was NOT in mind - just enough effort was put into it to make it work for demonstration purposes.

Contained within the diagram is enough information to connect your cheap, 3-volt laser pointer - just be sure to pay close attention to its positive and negative battery connections when you take it apart!

Also contained in this diagram is a very simple, low-performance receiver intended solely for across-the-room (or across-the-parking lot) testing of the transmitter to make sure that it works. It should be emphasized that this receiver is not at all intended for longer-distance use - say, more than a few hundred meters at most, and its performance can be spectacularly enhanced with the careful installation of a small magnifying glass lens with the phototransistor at its focus. Even when enhanced thusly, other optical receiver circuits will still run circles around it! A link to a web page describing a far more sensitive circuit may be found at the bottom of this page.

At this point I'll make a few comments about laser safety and legality:

Make certain that your "laser range" is end-stopped - that is, when the beam goes beyond the receiver it does not cross a road or have any likelihood of being intercepted by aircraft in flight or landing/taking off where they can dazzle and distract! In other words, the receive end should be against the side of a building or hill.

While cheap, red laser pointers are probably too weak to cause permanent eye damage, it's best not to stare into it or point it directly at people! A standard, cheap red laser pointer will, at its worst, probably just dazzle and maybe cause a brief headache or eye pain as well as a temporary loss of night vision. The farther you are away from it, the less dangerous it will be.

In some states and areas laser pointers are highly regulated or even illegal - including some U.S. and Australian states/localities - check your local listings!

It is NOT recommended that any but cheap, red laser pointers be used for this purpose. Why? First, they are the cheapest and secondly, they are fairly safe and low power. It's also worth considering that typical electronic detectors respond far better (e.g. are more sensitive) to red light than green or blue - not to mention there being less atmospheric attenuation at "red" wavelengths! Some of these "fancier" laser pointers of other colors have electronic circuits in them that can prevent them from being modulated effectively.

Figure 2:
Cheap laser pointer on a tripodClick on the image for a larger version.

One thing that you'll immediately notice about laser pointers is that
despite their name, they can be fiendishly difficult to aim them -
particularly as the distance increases! For this reason it's best to
contrive a means by which a camera tripod can be used to hold a laser
pointer - but even this can be tricky since even a fairly expensive
tripod is quite "touchy"! To the left you can see Ron's laser pointer mount with the pointer module itself being contained within a cheap project box from Radio Shack and connected by a short cable to the rest of the circuitry.

This brings up another point as well: Do not put both the modulator electronics and your laser pointer in the same box. By connecting them with a cable you will be able to make adjustments and turn the thing on and off without touching the tripod and possibly disturbing your carefully-aimed beam!

Another example of a laser pointer modified for such use may be seen on the right. When I got this pointer I couldn't see how I could remove the laser module without the possibility of damaging it so I simply used it as-is: A wooden dowel, the same diameter as AAA cells was used and at the inside end of the dowel was a small screw to which the minus (-) side connection was made. The connection to this screw was made via a wire laid in a shallow groove along the length of the dowel and the positive (+) side was connected to the case of the pointer itself by using some copper foil wrapped around the end of the dowel opposite the screw. The dowel was tack-glued into place, pushing against the internal battery spring and the laser's "on" button was simply taped down. The entire pointer was then "hot-glued" to a cheap project box that itself has inside it a 1/4"-20 bolt glued into place to allow attaching to a tripod mount while electrically insulating the laser pointer's positively-connected case from the tripod.

Figure 3:
Minimally-modified pointer on a tripod mount. This just happens to
be mounted atop an 8" astronomical telescope (a Celestron C8) with a equatorial mount which allows
precise aiming - and it also includes a telescope!Click on the image for a larger version.

A lot has been glossed over in this brief article - namely techniques about how to accomplish a laser communication over longer distances including links to descriptions of higher-performance gear and methods of precisely aiming - and if you are really interested, you can take a look at my page:

Under clear-air conditions on a line-of-sight path and using the very same lasers pictured above I've had a 2-way laser pointer to laser pointer communications on a 107 mile (173 km) path with fairly good signals. This was, of course, using high-performance receivers with orders of magnitudes better sensitivity than the one shown it Figures 1 and 4 on this page! In the "Using Laser Pointers..." link just above one can even find additional links to actual "off the air" recordings made via long-distance laser-pointer communications systems.

There are other problems with using lasers over distance, however, namely that of scintillation - the rapid fading or "twinkling" caused by the irregularities in the atmosphere. While this affects all types of light sources the combination of the coherent laser light and the small diameter of the beam as it exits the laser greatly exacerbates the problem - but that's a topic for another article!

A "Cheap" Optical Transceiver lens assembly - If you really want to improve your receive sensitivity, the best way to do this is with a large lens. This article describes how one would use foam-core poster board to make an assembly that would focus the distant light on the detector diode of an optical receiver.

Figure 4:Schematic diagrams of a simple (but deaf) receiver for testing and a simple PWM laser/LED transmitter, describedin the text above. This unit was designed by Ron, K7RJ and is shown in Figure 1, above.Click on the image for a larger version.